1. The Field of the Invention
This invention relates to photographic products and processes and particularly to diffusion transfer photographic products and processes.
2. Description of the Prior Art
Diffusion transfer photographic products and processes are known to the art and details relating to them can be found in U.S. Pat. Nos. 2,983,606; 3,415,644; 3,415,645; 3,415,646; 3,473,925; 3,482,972; 3,551,406; 3,573,042; 3,573,043; 3,573,044; 3,576,625; 3,576,626; 3,578,540; 3,569,333; 3,579,333; 3,594,164; 3,594,165; 3,597,200; 3,647,437; 3,672,486; 3,672,890; 3,705,184; 3,752,836; 3,857,865 and British Patent No. 1,330,524 all of which are incorporated here in their entirety.
Essentially, diffusion transfer photographic products and processes involve film units having a photosensitive system including at least one silver halide layer usually integrated with an image-providing material. After photoexposure, the photosensitive system is developed to establish an imagewise distribution of a diffusible image-providing material, at least a portion of which is transferred by diffusion to an image-receiving layer capable of mordanting or otherwise fixing the transferred image-providing material. In some diffusion transfer products, the transfer image is viewed by reflection after separation of the image-receiving element from the photosensitive system. In other products however, such separation is not required and instead, the transfer image in the image-receiving layer is viewed against a reflecting background usually provided by a dispersion of a white, light-reflecting pigment--such as titanium dioxide.
Diffusion transfer photographic processes and products providing a dye image viewable against a reflective background without separation are oftentimes referred to in the art as, "integral negative-positive film units" and such units are of two general types. Integral negative-positive film units of a first type as described for example in the above-referenced U.S. Pat. No. 3,415,644, include appropriate photosensitive layer(s) and image dye-providing materials carried on an opaque support, an image-receiving layer carried on a transparent support and means for distributing a processing composition between them. Photoexposure is made through the transparent base and image-receiving layer and a processing composition which includes a reflecting agent or pigment is then distributed between the image-receiving and photosensitive components. After distribution of the processing composition and before processing is complete, the film unit can be--and usually is--transported into light.
Integral negative-positive film units of a second type, as described for example in referenced U.S. Pat. No. 3,594,165, include a transparent support, carrying the appropriate photosensitive layers and associated image dye-providing materials, a permeable opaque layer, a permeable layer containing a light-reflecting pigment, an image-receiving layer viewable through the transparent support against the light-reflecting layer, and means for distributing a processing composition between the photosensitive layer and a transparent cover or spreader sheet. Additionally, integral negative-positive film units of this second type have means for providing a second opaque layer after photoexposure to prevent additional exposure of the photosensitive element. In film units of this second type, exposure is made through the transparent cover sheet. After distribution of the processing composition and installation of the second opaque layer this type of film unit can also be transported into light before processing is complete.
When integral negative-positive diffusion transfer film units of the types described are exposed to light for extended periods, the color reflection print has sometimes been observed to evidence discoloration of the white areas with time. This discoloration is considered to be caused by a decomposition or chemical change in ingredients of or ingredients associated with the reflecting layer. The decomposition or change is most likely initiated and/or accelerated by the action of light. The problem is most pronounced in those systems employing an opacification system of the type disclosed in U.S. Pat. No. 3,647,437. Such opacification systems include a light-reflecting pigment and an optical filter agent which is rendered colorless after a predetermined period by reduction of the environmental pH.